1. Field of the Invention
The present invention relates to liquid nitrogen temperature micro-wigglers. More particularly, the invention relates to micro-wigglers for free electron lasers (FEL). Still more particularly, the invention relates to the production of shorter radiation wavelengths with FEL devices.
2. The Prior Art
A FEL is a device in which an electromechanical or a magnetomechanical structure, called a wiggler or an undulator creates a magnetic field which periodically alternates in direction. Electrons passing through this magnetic field are deflected in a periodic fashion, and as a result emit characteristic optical radiation.
FEL's have the advantage, over other radiation producing devices, in that their radiation wavelength is variable by changing either the energy of the electron entering the wiggler or the strength of the magnetic field created by the wiggler.
There is a continuing need to produce shorter radiation wavelengths with FEL devices which are simpler and more compact in construction.
The radiated wavelength .lambda..sub.r of a free electron laser depends on the wavelength or spatial period .lambda..sub.w of the wiggler according to the following approximate formula ##EQU1## where .beta..sub.z is the average longitudinal velocity of the electrons, the traverse motion having been taken into account, divided by the speed of light c. For electrons moving near the speed of light, .beta..sub.z .apprxeq.1 the above formula reduces to the simpler result ##EQU2## where .gamma. is the relativistic factor.
The following table illustrates the dependence of .gamma. and radiated wavelength .lambda..sub.r in nanometers on electron energy in MeV for a wiggler of one centimeter period,
______________________________________ Energy in MeV .gamma. .lambda..sub.r Nanometers ______________________________________ 10 20.57 11817.78 25 49.92 2006.18 50 98.85 511.74 75 147.77 228.98 100 196.69 129.24 150 294.54 57.64 200 392.38 32.47 ______________________________________
A shorter wavelength radiation may obviously be produced for a fixed wiggler period by increasing the electron energy.
However, for electrons of a given energy, one can also produce shorter wavelength radiation by reducing the size of the wiggler period. Furthermore, reducing the wiggler period lowers the electron energy required to produce a given wavelength of radiation.
The shortest wiggler periods known to applicants are the subcentimeter superconducting planar undulator with a period of 0.88 cm at Brookhaven National Laboratory (BNL), and the slotted-tube, non-planar, 3 mm period micro-wiggler at Los Alamos National Laboratory (LANL).
The former BNL device has the disadvantage of the necessity for helium refrigeration and the latter LANL device has the disadvantage of pulsed operation and generation of extraneous fields, which tend to degrade FEL performance.